arXiv subjects

Chenyu Zhang

Publications and source records attributed to Chenyu Zhang.

95 records · Page 6Linked to original sources

arXiv2020

Control of polymorphism during epitaxial growth of hyperferroelectric candidate LiZnSb on GaSb (111)B

A major challenge for ferroelectric devices is the depolarization field, which competes with and often destroys long-range polar order in the limit of ultrathin films. Recent theoretical predictions suggest a new class of materials, termed hyperferroelectics, that should be robust against the depolarization field and enable ferroelectricity down to the monolayer limit. Here we demonstrate the epitaxial growth of hexagonal LiZnSb, one of the hyperferroelectric candidate materials, by molecular-beam epitaxy on GaSb (111)B substrates. Due to the high volatility of all three atomic species, we find that LiZnSb can be grown in an adsorption-controlled window, using an excess zinc flux. Within this window, the desired polar hexagonal phase is stabilized with respect to a competing cubic polymorph, as revealed by X-ray diffraction and transmission electron microscopy measurements. First-principles calculations show that for moderate amounts of epitaxial strain and moderate concentrations of Li vacancies, the cubic LiZnSb phase is lower in formation energy than the hexagonal phase, but only by a few meV per formula unit. Therefore we suggest that kinetics plays a role in stabilizing the desired hexagonal phase at low temperatures. Our results provide a path towards experimentally demonstrating ferroelectricity and hyperferroelectricity in a new class of ternary intermetallic compounds.

cond-mat.mtrl-sci↗

arXiv2019

High electrical conductivity in the epitaxial polar metals LaAuGe and LaPtSb

Polar metals are an intriguing class of materials that simultaneously host free carriers and polar structural distortions. Despite the name "polar metal," however, most well-studied polar metals are poor electrical conductors. Here, we demonstrate the molecular beam epitaxial (MBE) growth of LaPtSb and LaAuGe, two polar metal compounds whose electrical resistivity is an order of magnitude lower than the well studied oxide polar metals. These materials belong to a broad family of $ABC$ intermetallics adopting the stuffed wurtzite structure, also known as hexagonal Heusler compounds. Scanning transmission electron microscopy (STEM) reveals a polar structure with unidirectionally buckled $BC$ (PtSb, AuGe) planes. Magnetotransport measurements demonstrate good metallic behavior with low residual resistivity ($ρ_{LaAuGe}=59.05$ $μΩ\cdot$cm and $ρ_{LaAPtSb}=27.81$ $μΩ\cdot$cm at 2K) and high carrier density ($n_h\sim 10^{21}$ cm$^{-3}$). Photoemission spectroscopy measurements confirm the band metallicity and are in quantitative agreement with density functional theory (DFT) calculations. Through DFT-Chemical Pressure and Crystal Orbital Hamilton Population analyses, the atomic packing factor is found to support the polar buckling of the structure, though the degree of direct interlayer $B-C$ bonding is limited by repulsion at the $A-C$ contacts. When combined with insulating hexagonal Heuslers, these materials provide a new platform for fully epitaxial, multiferroic heterostructures.

cond-mat.mtrl-sci↗

arXiv2019

Electronically enhanced layer buckling and Au-Au dimerization in epitaxial LaAuSb films

We report the molecular beam epitaxial growth, structure, and electronic measurements of single-crystalline LaAuSb films on Al$_2$O$_3$ (0001) substrates. LaAuSb belongs to a broad family of hexagonal $ABC$ intermetallics in which the magnitude and sign of layer buckling have strong effects on properties, e.g., predicted hyperferroelecticity, polar metallicity, and Weyl and Dirac states. Scanning transmission electron microscopy reveals highly buckled planes of Au-Sb atoms, with strong interlayer Au-Au interactions and a doubling of the unit cell. This buckling is four times larger than the buckling observed in other $ABC$s with similar composition, e.g. LaAuGe and LaPtSb. Photoemission spectroscopy measurements and comparison with theory suggest an electronic driving force for the Au-Au dimerization, since LaAuSb, with a 19-electron count, has one more valence electron per formula unit than most stable $ABC$s. Our results suggest that the electron count, in addition to conventional parameters such as epitaxial strain and chemical pressure, provides a powerful means for tuning the layer buckling in ferroic $ABC$s.

cond-mat.mtrl-sci↗

arXiv2017

Imaging of Single La Vacancies in LaMnO$_{3}$

We report an approach for three-dimensional imaging of single vacancies using high precision quantitative high-angle annular dark-field Z-contrast scanning transmission electron microscopy (STEM). Vacancies are identified by both the reduction in scattered intensity created by the missing atom and the distortion of the surrounding atom positions. Vacancy positions are determined laterally to a unique lattice site in the image and in depth to within one of two lattice sites by dynamical diffraction effects. 35 single La vacancies are identified in images of a LaMnO3 thin film sample. The vacancies are randomly distributed in depth and correspond to a La vacancy concentration of 0.79%, which is consistent with the level of control of cation stoichiometry within our synthesis process (~1%) and with the equilibrium concentration of La vacancies under the film growth conditions. This work demonstrates an approach to characterizing low concentrations of vacancies with high spatial resolution.

cond-mat.mtrl-sci↗

arXiv2017

Multiparameter estimation with single photons

It was suggested in Ref. [Phys. Rev. Lett. 114, 170802] that optical networks with relatively inexpensive overhead---single photon Fock states, passive optical elements, and single photon detection---can show significant improvements over classical strategies for single-parameter estimation, when the number of modes in the network is small (n < 7). A similar case was made in Ref. [Phys. Rev. Lett. 111, 070403] for multi-parameter estimation, where measurement is instead made using photon-number resolving detectors. In this paper, we analytically compute the quantum Cramér-Rao bound to show these networks can have a constant-factor quantum advantage in multi-parameter estimation for even large number of modes. Additionally, we provide a simplified measurement scheme using only single-photon (on-off) detectors that is capable of approximately obtaining this sensitivity for a small number of modes.

quant-ph↗